High-quality meta-analyses, systematic reviews of RCTs, or RCTs with a very low risk of bias.

1+

Well-conducted meta-analyses, systematic reviews, or RCTs with a low risk of bias.

1−

Meta-analyses, systematic reviews, or RCTs with a high risk of bias.

2++

High-quality systematic reviews of case control or cohort or studies High-quality case–control or cohort studies with a very low risk of confounding or bias and a high probability that the relationship is causal.

2+

Well-conducted case control or cohort studies with a low risk of confounding or bias and a moderate probability that the relationship is causal.

2−

Case control or cohort studies with a high risk of confounding or bias and a significant risk that the relationship is not causal.